1. Field of the Invention
The present invention concerns a method implemented in a computer (processor) to determine the conductor structure of a gradient coil of a magnetic resonance device.
2. Description of the Prior Art
In magnetic resonance tomography, gradient coils are used for slice selection and for spatial coding of the signal. For this purpose, a magnetic resonance imaging device typically has three gradient coils in the form of a gradient coil arrangement, wherein the gradient fields generated by the individual gradient coils are orthogonal to one another. Furthermore, it is typical that the gradient fields generated by the gradient coils proceed perpendicularly to or parallel to the magnetic field of the basic field magnet. These gradient fields generated by the gradient coils can be considered as a type of basic gradient fields. Arbitrary gradient fields can then be generated in arbitrary spatial directions by superimposing the individual basic gradient fields.
In closed systems that are designed cylindrically as a whole and also have a cylindrical central bore, all larger structures (such as gradient coils, cryostat and vacuum container) circularly surround the bore in an essentially concentric design.
Although given such an arrangement the conductor structure can be located on a cylindrical surface, the conductor structure of a gradient coil does not completely surround the bore.
The gradient field of a gradient coil that is generated by means of the conductor structure should simultaneously satisfy multiple criteria. The gradient field should exhibit an optimally good linearity in the examination area. Only with a linear gradient field can a precise slice selection and a useful spatial coding be achieved. Second, the gradient field generated by the gradient coil should exhibit a specific minimum strength. The achievable resolution of the image data acquired with the magnetic resonance device depends on the strength of the gradient field. The higher the gradient strength, the greater the achievable resolution capability. Third, the switching time required for activating the gradient fields should be optimally short. In fast imaging sequences, the switching time affects the echo time to a significant degree, which is why the gradient coil should be able to bring the gradient field to its desired amplitude as quickly as possible. Last but not least, the gradient coil should generate the gradient field at best only within the examination volume while outside of this volume no fields should be generated in the optimal case.
It is the last requirement that, understandably, cannot be satisfied in reality. This is because, a gradient coil generates eddy currents in the surrounding metallic structures, and these eddy currents lead to oscillations and therefore also to the development of noise, for example. In order to minimize the generation of eddy currents in structures outside of the gradient coil, it is known to use a technique called active shielding. This technique makes use located of coils that are arranged outside of the gradient coil arrangement. A current of the same strength as that through the gradient coils flows through these compensation coils, so the respective magnetic fields of the compensation coils and the gradient coils essentially cancel one another in the outer region. Through such active shielding, the formation of eddy currents in metal structures of a magnet (such as a cryoshield and vacuum tank) can be minimized but not completely prevented. Therefore, as before it is sought to keep the eddy currents generated by the gradient coil in the metallic structures surrounding the gradient coil as small as possible.
Moreover, a particular problem is to satisfy this criterion in measurement (data acquisition) protocols in which the gradient coils must be switched extremely rapidly, so the switching frequency generates eddy currents at a frequency in the range of the frequencies of the natural oscillation modes of the metallic structures of the magnetic resonance device. Such critical switching cycles can be prevented only by specific parameters controlling the data acquisition not being permitted to occur. For example, this means that the commonly-used echo-planar imaging (EPI) sequence cannot be operated with specific repetition times or echo times in a magnetic resonance device of a specific type. The echo times and repetition times vary somewhat depending on the design of the magnetic resonance device. The operation of the magnetic resonance devices is accordingly limited.
Due to the multiple criteria to be satisfied, there are conflicts in the optimization of the conductor structure of the gradient coil, to the effect that all requirements cannot be optimally satisfied simultaneously. The design of a gradient coil arrangement is thus a classical multiple goal optimization problem, wherein under the circumstances a gain (for example in the linear of the gradient field generated by the gradient coil) is purchased with stronger eddy currents.